The present invention relates to the field of casting, and more particularly to a mold for casting, and also to methods of fabricating shell molds, and to methods of casting using a mold.
In the description below, the terms “high”, “low”, “horizontal”, and “vertical” are defined by the normal orientation of such a mold while metal is being cast into it.
So-called “lost-wax” or “lost-pattern” casting methods have been known since antiquity. They are particularly suitable for producing metal parts that are complex in shape. Thus, lost-pattern casting is used in particular for producing turbine engine blades.
In lost-pattern casting, the first step normally comprises making a pattern out of a material having a melting temperature that is comparatively low, such as for example out of wax or resin. The pattern is itself coated in refractory material in order to form a mold, and in particular a mold of the shell mold type. After removing or eliminating the material of the pattern from the inside of the mold, which is why such methods are referred to as lost pattern casting methods, molten metal is cast into the mold in order to fill the cavity that the pattern has formed inside the mold by being removed or eliminated therefrom. Once the metal has cooled and solidified, the mold may be opened or destroyed in order to recover a metal part having the shape of the pattern. In the present context, the term “metal” should be understood to cover not only pure metals but also, and above all, metal alloys.
In order to be able to make a plurality of parts simultaneously, it is possible to unite a plurality of patterns in a single assembly in which they are connected together by a tree that forms casting channels in the mold for the molten metal.
Among the various types of mold that can be used in lost-pattern casting, so-called “shell” molds are known that are formed by dipping the pattern or the assembly of patterns into a slip, and then dusting refractory sand onto the pattern or the assembly of patterns coated in the slip in order to form a shell around the pattern or the assembly, and then baking the shell in order to sinter it and thus consolidate the slip and the sand. Several successive operations of dipping and dusting may be envisaged in order to obtain a shell of sufficient thickness prior to baking it. The term “refractory sand” is used in the present context to designate any granular material of grain size that is sufficiently small to satisfy the desired production tolerances, that is capable, while in the solid state, of withstanding the temperature of the molten metal, and that is capable of being consolidated into a single piece during baking of the shell.
In order to obtain particularly advantageous thermomechanical properties in the part produced by casting, it may be desirable to ensure that the metal undergoes directional solidification in the mold. The term “directional solidification” is used in the present context to mean that control is exerted over the nucleation and the growth of solid crystals in the molten metal as it passes from the liquid state to the solid state. The purpose of such directional solidification is to avoid the negative effects of grain boundaries within the part. Thus, the directional solidification may be columnar or monocrystalline. Columnar directional solidification consists in orienting all of the grain boundaries in the same direction so that they cannot contribute to propagating cracks. Monocrystalline directional solidification consists in ensuring that the part solidifies as a single crystal, so as to eliminate all grain boundaries.
In order to obtain such monocrystalline directional solidification, the mold typically presents, beneath the molding cavity, a starter cavity that is connected to the molding cavity by a selector channel, as disclosed by way of example in French patent FR 2 734 189 and U.S. Pat. No. 4,548,255. While the metal is solidifying in the mold, the mold is cooled progressively starting from the starter cavity so as to cause crystals to nucleate therein. The role of the selector channel is firstly to favor a single grain, and secondly to enable the single grain to advance towards the molding cavity from the crystallization front of this grain that nucleated in the starter cavity.
A drawback of that configuration is nevertheless that of ensuring that the mold has mechanical strength, in particular when the mold is of the so-called “shell mold” type, made up of relatively thin walls around the cavities and channels that are to receive the molten metal, since the molding cavity occupies a high position above a starter cavity that is normally smaller. For this purpose, it is common practice, as shown in U.S. Pat. No. 4,940,073, to incorporate support rods in the mold.
Nevertheless, such support rods, which penetrate into the starter and molding cavities can interfere with grain nucleation and propagation.